The paralleling of amplifiers for higher combined power is well known. U.S. Pat. No. 5,222,246, issued Jun. 22, 1993 in the name of Wolkstein describes the paralleling of radio-frequency (RF) amplifiers located on a spacecraft, to achieve higher power. The term RF includes microwave, millimeter-wave, and, generally, signals at frequencies above sonic frequencies. As described in the Wolkstein patent, an RF signal source is coupled to the input of a power splitter, which produces two equal-amplitude signals at its output ports. A hybrid power combiner includes two input ports, and sum and difference output ports. A first channel connects one output port of the splitter to a first input port of the power combiner, and a second channel connects the second output port of the splitter to the second input port of the combiner. As described in the Wolkstein patent, phase shifts between amplifiers being paralleled must be kept within about ten or fifteen degrees, and preferably should be within about five degrees. Simple aging can change the phase characteristics of an RF amplifier, and travelling-wave amplifiers tend to be highly sensitive to their energizing voltage. Since each individual or elemental amplifier of a paralleled amplifier pair may be energized by a separate electronic power controller (EPC), their applied voltages may drift, causing substantial drift of the relative phase during normal operation. A controllable phase shifter coupled in at least one of the channels allows for correction of the phase of the channel in which it resides, and can be adjusted to maximize the combined signal power at the sum output port of the combiner, to thereby compensate for drift. The preferred embodiment uses two phase shifters, one in each channel. In order to provide sensitive control of the phase shifter, an output signal is taken from the null output port of the combiner, processed, and fed back to the phase shifter, in a manner which minimizes the signal at the null port, and therefore maximizes the combined signal at the sum port. In one embodiment of the arrangement of the Wolkstein patent, the signal splitter is a hybrid, and in another embodiment, the signal splitter is resistive, and a compensating phase or delay adjustment is introduced into one of the legs to provide the desired phase relationship for the output hybrid combiner.
In the context of amplifiers which cannot conveniently be accessed for repair, some redundancy schemes may be used. It is well known that RF amplifiers tend to age, and that their characteristics change with age. Thus, the gain of an RF amplifier will tend to decrease with increasing operating time. Similarly, the phase shift may also change with time, as described, for example, in U.S. Pat. No. 4,701,716, issued Oct. 20, 1987 in the name of Poole. In the context of a pair of paralleled amplifiers, it has been the practice to substitute a paralleled amplifier for a defective paralleled amplifier. Thus, in a communications spacecraft having, say, twenty paralleled amplifiers in twenty separate channels, twenty additional paralleled amplifiers would be necessary as spares in order to have two-for-one redundancy, thereby requiring that forty paralleled amplifiers be fabricated and carried by the spacecraft in order to have redundancy for twenty paralleled amplifiers. It is also well known that minimization of excess weight is an important consideration in spacecraft systems. The twenty paralleled amplifiers provided for redundancy of necessity add substantial additional weight.